Conventionally, there have been proposed and used many types of filtrating apparatuses for filtrating various polluted substances, smelled (nasty) substances and impurities which are unsuitable for drinking to obtain drinking water from river water, seawater and drained water. Further, there are commercially available water purifying devices and the like for removing a nasty smell (foul smell) and a bad taste caused by organic substances, inorganic substances and the like contained in well water and tap water in a slight amount. Many of the water purifying devices use a filter unit.
Additionally, a filtrating process is indispensable to food industries to remove solid substances and impurities from fermented products for making beer, wine, Japanese sake, soy sauce etc. and to remove solid substances and impurities to obtain sauce, juice, fruit juice, cooking oil etc. from a food material.
In particular, since drinking water, liquors, fruit juice, juice, sauce, cooking oil etc. are used for drinking or eating, a very high performance is required of a purifying apparatus, thus a high performance is also required of a filtrating device.
Further, with the recent fitness boom, many persons enjoy swimming as an exercise of every part of the body regardless of age and sex and the number of persons visiting swimming pools is increasing.
With this trend, there is an increasing interest in the sanitary state of the swimming pool and in particular in the quality of water to enjoy swimming more safely and sanitarily. Thus the requirements for a purifying apparatus by which the quality of water in the swimming pool is established is made more and more strict and thus the performance of a filtrating device used meet this requirement must be of the highest level.
Further, filtrating devices are now being widely used in processing water from a low purity level to a high purity level in such fields as the water purification in zoos, aquariums and the like, the pre-processing in the manufacture of pure water used for the production of semiconductors and the desalination of seawater, the quahouse and 24-hour open bath, water processing for an intermediate water supply, decolorization processing required by the environmental standards and the like.
Many porous ceramic filters are used as the filter element of the filtrating apparatus. The porous ceramic filter (hereinafter, referred to as a ceramic filter) has very fine three-dimensional filtrating spaces (holes) which are very preferable to the filtration of water in the swimming pool and in particular in the warm water swimming pool and can preferably filtrate organic substances such as oil and the like, structures, protein, bacteria, viruses and the like discharged from human bodies.
The purifying capability of a ceramic filter is determined by the size of micropores used in filtration, wherein the smaller the micropore size, the more excellent the purifying capability of the filter. Further, when the ease of cleaning the filter by reverse washing and the like is taken into consideration it is preferable that impurities and the like in a liquid to be purified are captured on the surface of the ceramic filter.
Therefore, it is preferable that a ceramic filter is particularly close on the surface thereof through which a liquid to be purified flows in. However, since the ceramic filter is usually manufactured by preparing a compound made by mixing and kneading fine particles with a binder, forming the compound to a molded product having a predetermined shape by compacting and molding it and sintering the molded product, manufacturing a ceramic filter having the above structure is not feasible.
To manufacture a ceramic filter having the above structure (having an average particle size which is made smaller toward the surface thereof), it is contemplated to manufacture a ceramic filter having a preferable particle distribution in which the filter surface through which a liquid to be purified flows into is particularly close, in such a manner that a compound (slurry) composed of still more ultrafine particles is coated on the surface of a molded product, dried and then sintered to thereby form a layer composed of the ultrafine particles on the surface of the filter.
However, this manufacturing method is troublesome and time consuming. Additionally, the manufacturing cost of the ceramic filter is high. Moreover, the slurry must be repeatedly coated and dried many times to sufficiently form the layer composed of the ultrafine particles on the surface. Further, the thus made ceramic filter has a two-layered surface structure or a multi-layered structure as FIG. 9 schematically shows. Thus, the coated layer located on a base member is not perfectly arranged as an integral part thereof. Therefore, the coated thin layer is easily exfoliated while the filter is subjected to reverse washing or executes filtration and the exfoliation expands. As result, the purifying capability of the exfoliated portion is lowered and the portion cannot exhibit a desired purifying capability, by which serious problems such as short life of the ceramic filter and the like are caused in practical use.
On the other hand, since the conventional ceramic filter is made more coarse toward the surface thereof, when it is used for filtration without using a filtration assistant, impurities enter from a surface to the filter deep interior and cause clogging. Thus, it is very difficult to clean (reversely wash) the filter.
To cope with this problem, a filtration apparatus using the ceramic filter forms the layer of a fibrous filtration assistant such as pulp fibers, asbestos or the like or the layer of fine particle filtration assistant such as diatom earth, lime or the like, that is, a so-called filtration assistant layer on the surface of the filter through which water to be filtrated flows in to thereby make the surface of the ceramic filter to a close state for the prevention of clogging of the ceramic filter and cleaning effected by reverse washing easy. In this instance, a problem arises in that the filter cleaning becomes a time-consuming job.
Further, it is desired that a filter fulfills other functions such as, for example, heating, sterilization, control of a specific component, pre-processing for a special process used to the manufacture of pure water, the desalination of seawater, and the like.